Carbon dioxide-based chromatography systems, for example, SFC (supercritical fluid chromatography) systems, employ a compressible mobile phase as a carrier fluid. Separations performed by such chromatography systems require independent control over the mobile phase flow rate and system pressure in order to maintain a constant linear velocity through the separation column For chromatography systems using packed bed, analytical-scale separation columns, a backpressure regulator (BPR) often provides this decoupling of mobile phase flow rate and system pressure.
In addition, a common practice in chromatography systems with analytical-scale separation columns is to split the compressible mobile phase flow. Splitting the compressible mobile phase flow overcomes two system limitations. One is that BPR can contribute significant dead (i.e., extra) volume to the chromatography system, significantly broadening the resulting chromatographic peaks and making detection downstream of the BPR undesirable; splitting provides a path by which to direct a portion of the mobile phase flow towards a detector, a path without the BPR. The second is that detectors, such as mass spectrometers (MS) and flame ionization detectors (FID), among others, are designed to achieve an optimum response within a tight window for the mobile phase flow rate; splitting the compressible mobile phase enables the chromatography system to provide the optimum flow rate consistently to the detector over a wide range of total system flow rates governed by the separation column. Detection downstream of the BPR is further undesirable because the mobile phase has decompressed after the BPR, leaving no appreciable mobile phase density available for transporting the analyte. In this instance, high molecular weight and/or low volatility analytes will precipitate out of the mobile phase without ever reaching the detector.
Typically, the chromatography system achieves this splitting with a fixed restrictor installed in a tee fitting residing in the compressible mobile phase stream. The fixed restrictor routes a portion of the total mobile phase flow rate towards the detector for detection. In addition, the tee fitting routes a majority portion of the mobile phase flow rate to the BPR for maintaining and controlling system pressure.
This fixed restrictor configuration runs into problems, however, when the mobile phase density or composition changes during the course of a separation. Altering the mobile phase density or composition as the separation progresses (gradient separations) are commonly employed techniques for improving the peak capacity of the separation column Because the viscosity of the mobile phase changes with programmed changes to the mobile phase density or composition, the flow rate through the fixed restrictor changes, too. This change in flow rate causes a change in the split ratio (i.e., the ratio of the total mobile phase flow rate delivered by the pump to the minority portion mobile phase flow rate measured at the detector) as the separation progresses (a larger split ratio means a smaller portion of the mobile phase is directed to the detector). Therefore, the split ratio for latter eluting peaks differs from that for earlier eluting peaks. This difference in the split ratio creates a non-linearity, which poses a problem for quantitation of all peaks in the gradient separation. In addition to changes in viscosity, the split ratio changes simply by increasing the BPR pressure (i.e., pressure at the head of the split restrictor). The pump maintains a constant flow rate of the mobile phase across the column as the BPR increases the pressure. The fixed restrictor allows more mobile phase to pass to the detector as pressure increases and, therefore, the split ratio drops (i.e., a larger portion of the analyte is directed to the detector).